1. Field of the Invention
The invention relates to a forming tool, advantageously made from a metal with a forming contact surface and with one or several canals which are integrated in the tool and through which a fluid flows.
2. Description of the Related Art
When parts are moulded by injection from thermoplastic, duroplastic or elastomeric materials, the material, which has been plasticized beforehand, is introduced into the cavity of a mould. For making up the cavity, such a mould is composed of several forming tools which are separated from each other in order to remove the part cooled in the cavity. The surfaces of the forming tools, which are in contact with the moulding plastic material, are called contact surfaces within the context of these realisations. The image of these contact surfaces defines the surface of the part. The moulding precision of the part surface can be spoiled by defaults, for example by glued joint grooves, brightness differences or mottling and mark formation. Such surface defaults can be generated by irregular contact surface differences for the forming tools.
For improving the moulding precision for the plasticized material introduced into the cavity of a mould, for example a fused plastic material, it is known to increase the tool wall temperature, thus the temperature of the contact surfaces of the forming tools, so that the flowability of the fused plastic material introduced into the cavity is improved at its edge region.
For regulating the temperature of such forming tools, these comprise temperature regulation canals through which fluids, regulated at an appropriate temperature, for example water or oil, flow in order to maintain the forming tool at a determined temperature or to heat it up to a determined temperature. If such forming tools must have in determined regions a higher temperature, different from the basic temperature, electric heating cartridges or electric cooling elements, for reducing the temperature locally, are further arranged in recesses in the forming tool. Forming tools are also known in which the cooling elements are used for performing a definite cooling of the fused plastic material and thus for reducing the cooling process, therefore the cycle time.
In DE 199 29 731 A1 it is described a forming tool which can be subjected to an induction heating for introducing heat into the cavity and which is made, for this purpose, from an electrically conductive material. In U.S. Pat. No. 4,439,492 it is described a forming tool in which inductive heating inserts for heating the contact surfaces are inserted into the cavity. After heating the contact surfaces of the forming tools, the mould is opened and the heating insert is removed before closing again the mould in order to perform then the injection moulding process as such. The design of this forming tool is based on the idea that the insertion of inductive heating inserts enables to heat not all the forming tool but only the contact surfaces and the forming tool regions close to them. In this manner, the cycle times can be reduced. A control of the temperature regulation during the injection moulding process is not possible with this design.
These already known means enable to heat forming tools. The cycle times during the injection moulding are decisively determined for hardening the material introduced in a plasticized state into the mould cavity. For reducing the cycle times, forming tools have been developed in which it is provided, as well as the temperature regulation canals for heating the forming tool, additional temperature regulation canals through which a cooling agent can flow. Such a forming tool can thus be heated and cooled. The inconvenient of such a conception of a forming tool is however its relative inertial behaviour. In order to eliminate this inconvenient, it is proposed in DE 102 57 129 to heat the contact surfaces of a forming tool, i.e. it is proposed that the inductor used for heating the first contact surface is integrated in a first forming tool which is placed opposite the contact surface to be heated of a second forming tool. Thus, in this already known forming tool, the inductor is separated by the cavity from the contact surface to be heated. In order to be able to heat a contact surface in this manner, it is necessary that the contact surface to be heated of the forming tool is made from an appropriate material. In most cases, this material differs from the material of the other forming tools used for forming the cavity. For surface defaults not to appear at the transition between the material necessary for the heating process and the contact surface material of other cavity limitations these transitions must be very carefully realised or the entire surface around the cavity is made from this material. Moreover, the method described in this document can only be used with determined tool geometries.